Corn in Mississippi
When should irrigation of corn be terminated?
Irrigation should be continued long enough to supply plant moisture needs until physiological maturity is reached. Physiological maturity is signified by the formation of a dark abscission layer, referred to as the "black layer" at the base of kernels. This abscission layer forms when hard starch accumulation completes its progression from the top to bottom of a kernel. Seed weight accumulation is complete at this time. Physiological maturity usually occurs about 20 days after dent stage or 60 days after silking. Mississippi grown corn planted during the suggested planting dates normally reaches physiological maturity from late July to early August, depending primarily upon location, planting date and climatic conditions during the season.
Hard starch begins forming at the kernel tip when denting occurs. This hard starch layer gradually progresses to the kernel base over the next 20 days. This progression may be checked by squeezing kernels or observing the "milkline" on kernels. This progession is very important to irrigation scheduling. The milkline is the borderline between the bright, clear yellow color of the seed coat overlying the hard starch layer, compared to the milky, dull yellow seed coat overlying the hard dough layer. (Photo of a milk-line) An abscission layer forms when the hard starch layer reaches the kernel base. Physiological maturity is signified by this black or brown abscission layer which is referred to as the "black layer". This abscission layer cuts off water and dry matter transfer into the kernel. Kernels usually have a moisture content of 28-35% at this stage. The black layer may be found by gently scraping away the seed coat at the kernel base on the side opposite the embryo to expose the abscission layer. (Photo of the "black layer) The black layer formation occurs progressively from kernels at the tip of the ear to the base.
A common problem is early termination of irrigation. Premature irrigation termination will accelerate maturity, prohibiting kernels from reaching their full potential size and weight. Although kernels appear somewhat mature and corn water use begins declining at the dent stage, this is too early to terminate irrigation. Potential kernel weight is only about 75% complete at the dent stage. Thus, termination of irrigation at the dent stage can reduce grain yields as much as 15-20% when hot, dry conditions persist.